Conventionally, for the gasoline engines equipped to automobiles or the like, direct injection gasoline engines have been employed from the viewpoint of an improvement in fuel efficiency, etc. However, in such direct injection gasoline engines, particulate matter (hereinafter referred to as “PM”) is produced; therefore, investigation has progressed for technology providing an exhaust purification filter (Gasoline Particulate Filter (hereinafter abbreviation of “GPF” may be used)) which captures PM in the exhaust passage of the gasoline engine, accompanying the strengthening of emission regulations in recent years.
In addition, in the exhaust passage of gasoline engines, in order to purify CO, HC and NOx contained in the exhaust, a catalytic converter configured by loading a three-way catalyst (hereinafter, abbreviation of “TWC” may be used) onto a honeycomb support body has been provided. In particular, a plurality of catalytic converters for satisfying the required purification performance has been arranged in series in the exhaust passage in recent years. For this reason, newly providing a GPF to the exhaust passage in addition to this plurality of catalytic converters is not preferable from the viewpoint of pressure loss and cost.
Therefore, technology has been proposed of imparting three-way catalyst function by the TWC in addition to the PM capturing function possessed by a filter substrate, by way of loading the TWC onto the filter substrate of the GPF (for example, refer to Patent Document 1). According to this technology, it is anticipated as being possible to eliminate the problems of pressure loss and cost, by integrating the GPF and TWC. In other words, by imparting the functions of TWC to the GPF, since it is possible to reduce the number of catalytic converters that should be provided in the exhaust passage by this amount, it is possible to suppress an increase in pressure loss and cost in the exhaust purification system overall, and thus is logical.
Patent Document 1: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2013-500857
Herein, the challenges of an exhaust purification system established by arranging a catalytic converter and GPF in the exhaust passage in series will be considered in detail. So long as imparting the TWC functions to the GPF in the aforementioned way, it is anticipated as being possible to reduce the number of TWCs required for achieving the exhaust purification performance required by this amount. However, in order to ensure the PM capturing function in the filter substrate used in the GPF, contrary to the honeycomb support used in the catalytic converter, sealing is provided thereto. For this reason, even if TWC is not loaded, the pressure loss is high, and the filter substrate of the GPF cannot support as much TWC as the honeycomb support. In other words, the GPF to which the TWC functions are imparted cannot simply become a replacement for a catalytic converter. Therefore, with the exhaust purification system established by arranging a catalytic converter and GPF in series, when trying to ensure the exhaust purification performance of the overall system, it cannot be avoided to load a great amount of TWC onto the filter substrate of the GPF, and the increase in pressure loss of the overall system when doing so is a concern.
When trying to load TWC onto the filter substrate of the GPF in this way, the coexistence of pressure loss and exhaust purification performance is difficult. For this reason, the development of a TWC suited to the filter substrate has been desired. For example, with the technology of Patent Document 1, the NOx purification performance may greatly decline in particular in the case of using TWC containing Rh as the catalytic metal. For example, technology having a two-layer structure of an Rh layer and Pd layer as the TWC having superior three-way purification function has been known; however, it leads to great pressure loss when loading TWC of such as two-layer structure onto the filter substrate of the GPF. For this reason, it has been considered to load TWC of a single-layer structure obtained by mixing Rh and Pd on the filter substrate. However, in this case, Ba which is normally added to the Pd layer for the Pd deterioration suppression and improvement in NOx absorption performance will contact or be in proximity to Rh. Given this, Rh is oxidized to form an oxide by the electron-donating action of Ba, and as a result of the NOx reducibility of Rh declining, there is a problem in that a NOx purification performance as much as the GPF cannot be achieved.
The present invention has been made taking the above into account, and an object thereof is to provide an exhaust purification system of an internal combustion engine which can exhibit superior exhaust purification performance, while suppressing an increase in the pressure loss of the overall system, in an exhaust purification system established by arranging a catalytic converter and GPF in the exhaust passage in series.